The invention relates to magnetic devices that exploit the dependence of a physical property, such as resistance, emission current or optical behavior, on the relative magnetization direction of the device. Such devices include, without limitation, magnetic memory cells, magnetic random access memories (MRAM), spin transistors, and near-field magneto-optical applications. More specifically, the invention relates to techniques for reducing the writing current required for the device to switch states, and increasing the magnetization stability and power gain of the device.
A magnetic memory cell is a non-volatile memory that typically includes a portion of anisotropic magnetoresistive (AMR), colossus magnetoresistive (CMR), giant magnetoresistive (GMR) or magnetic tunnel junction (MTJ) material cooperating with electronic read and write circuits. The device employs a magnetic vector direction to store memory states, and a magnetoresistive effect for memory readout. In a GMR device, two or more layers of ferromagnetic material are separated by a thin metallic layer. An MTJ device has two ferromagnetic layers separated by a thin electrical insulator that acts as a tunneling barrier. Although these two types of devices operate according to different physical principles, in both types of memory cells, the electrical resistance to current flow through the device is substantially different if the two ferromagnetic layers are magnetized in a common direction (parallel magnetization) compared to when they are magnetized in opposite directions (antiparallel magnetization). An AMR device or CMR device has a single ferromagnetic material that behaves according to the AMR or CMR property, respectively. In both types of memory cells, the electrical resistance to current flow through the device is also substantially different depending on the magnetization direction of the ferromagnetic material.
In a typical GMR or MTJ magnetic memory cell, one layer of ferromagnetic material is fixed (“pinned”) in one direction and the second layer, referred to herein as the active layer, is made to change its magnetization in response to an applied external magnetic field over a certain threshold, named coercivity or coercive field or switching field. According to the direction of the magnetic vectors in the active layer of the device, states are stored, for example, the parallel direction can be defined as a logic “0”, and the antiparallel direction can be defined as a logic “1”, or vice-versa. If the magnetic memory cell is an AMR device or a CMR device, the single magnetic material as the active layer can change its permanent magnetization in response to an applied external magnetic field greater than the coercivity. According to the direction of the magnetic vectors in the active layer of the device, states are stored, similar to GMR or MTJ device, for example, the rightward direction can be defined as a logic “0”, and the leftward direction can be defined as a logic “1”, or vice-versa. The active layer of the device maintains these states even after removal of the external magnetic field. The state stored in the device can be read by a sense line which passes current through the device, since the different electrical resistance exhibited by the device due to the different magnetic vector directions in the active layer cause a different voltage output in the sense line.
A typical MRAM device includes an array of magnetic memory devices or cells. In one arrangement, word lines extend along rows of the memory cells and bit lines extend along columns of the memory cells. Each memory cell is located at a cross point of a word line and a bit line. The magnetization orientation of each memory cell (parallel or anti-parallel) may be changed by supplying currents to a word line and a bit line crossing the selected memory cell. When current flows through a bit line or a word line, it generates a magnetic field around the line. The arrays are designed so that each conductive line supplies only part of the field needed to reverse the magnetization of the active layer of the storage cells. Switching occurs only at those intersections where both word and bit lines are carrying current. Neither line by itself can switch a bit; only those cells addressed by both bit and word lines can be switched.
However, switching of the memory cells is not always reliable. Sometimes, the combined magnetic fields might not cause a memory cell to switch reliably and perfectly from parallel to anti-parallel or vice-versa for a GMR or MTJ device, or from right to left or vice-versa for an AMR or CMR device, due to such factors as the domain wall rotation, domain nucleation, interaction between bits, or the shape anisotropy. This problem can typically be solved by increasing crystal anisotropy, coercivity or the aspect ratio of the memory cells, but these solutions can lead to another problem: the amount of current for switching the memory cells is also increased. Increasing the amount of current increases the amount of power consumed by the MRAM device. Increasing the amount of current also results in larger bit and word lines and write circuits to handle the higher currents, resulting is a larger, more expensive MRAM device. Alternatively, the MRAM device could lose writing reliability by the electron migration effect.
Other problems with conventional MRAM arrays arise because of the need for ever-increasing levels of integration. But as memory cell size is reduced, the magnetic field required to write to the cell is increased, making it more difficult for the bit to be written. Again, larger writing currents can provide the required field strengths, but at the expense of larger conductors and write circuits. In addition, as conducting lines are made closer together, the possibility of cross talk between a conducting line and a cell adjacent to the addressed cell is increased. If this happens repeatedly, the stored magnetic field of the adjacent cell can erode, and the information in the cell can be rendered unreadable.
Therefore, a need exists to reduce the writing current required for switching a magnetic device, to improve switching reliability and stability, and to better isolate the cells of the array from each other.